U.S. patent number 6,890,680 [Application Number 10/078,728] was granted by the patent office on 2005-05-10 for modified diffusion layer for use in a fuel cell system.
This patent grant is currently assigned to MTI MicroFuel Cells Inc.. Invention is credited to Gerhard Beckmann, Shimshon Gottesfeld, Frank W. Kovacs, Paul F. Mutolo, Xiaoming Ren.
United States Patent |
6,890,680 |
Beckmann , et al. |
May 10, 2005 |
Modified diffusion layer for use in a fuel cell system
Abstract
A fuel cell diffusion layer providing a preferential path by
which liquid reactants or byproducts may be supplied to or removed
from a direct oxidation fuel cell is described. The modified
diffusion layer will be typically on the cathode side of the fuel
cell and its use is to eliminate or minimize flooding of the
cathode diffusion layer area, which is a performance limiting
condition in direct methanol fuel cells. In accordance with one
embodiment of the invention, the diffusion layer includes a
substrate that is coated with a microporous layer. A pattern may be
embossed into the diffusion layer, to create preferential flow
paths by which water will travel and thereby be removed from the
cathode catalyst area. This avoids cathode flooding and avoids
build up of potentially destructive pressure by possible cathodic
water accumulation. This also provides a means for collecting
cathode water for redirection In accordance with another aspect of
the invention, the preferential path is established by applying a
thicker microporous layer to the carbon cloth or carbon paper and
drying it in such a fashion so that when it dries, the surface of
the microporous layer cracks to provide the pathways.
Inventors: |
Beckmann; Gerhard (Altamont,
NY), Ren; Xiaoming (Guilderland, NY), Mutolo; Paul F.
(Albany, NY), Kovacs; Frank W. (Waterford, NY),
Gottesfeld; Shimshon (Niskayuna, NY) |
Assignee: |
MTI MicroFuel Cells Inc.
(Albany, NY)
|
Family
ID: |
27732892 |
Appl.
No.: |
10/078,728 |
Filed: |
February 19, 2002 |
Current U.S.
Class: |
429/414; 502/101;
429/513; 429/480; 429/481; 429/482; 429/484 |
Current CPC
Class: |
H01M
8/1009 (20130101); H01M 4/96 (20130101); H01M
8/1004 (20130101); H01M 4/8807 (20130101); H01M
4/8605 (20130101); H01M 4/8817 (20130101); Y10T
428/30 (20150115); Y02E 60/50 (20130101); H01M
4/92 (20130101); Y02E 60/523 (20130101) |
Current International
Class: |
H01M
4/96 (20060101); H01M 8/10 (20060101); H01M
4/86 (20060101); H01M 4/88 (20060101); H01M
4/90 (20060101); H01M 4/92 (20060101); H01M
004/96 (); H01M 008/10 (); H01M 004/88 () |
Field of
Search: |
;429/30,33,34,42,44
;502/101 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Fuel Cell", 1992 Fuel Cell Seminar, Program and Abstracts, pp.
233-236, 461-464. .
"Miniaturized Fuel Cells for Portable Power", Helen L. Maynard and
Jeremy P. Meyers, Lucent Technologies, 2000. .
"Pocket-size PEMs", Paul Sharke, Mechanical Engineering. .
"Polymer Electrolyte Fuel Cells as Potential Power Source for
Portable Electronic Devices", Shinshon Gottesfeld and Mahlon S.
Wilson, pp. 486-517. .
Ren, Xiaoming, et al. Method Cross-Over in Direct Methanol fuel
Cells, Electrochemical Society Proceedings vol. 95-23, Oct. 1995,
pp. 284-293..
|
Primary Examiner: Le; Hoa Van
Attorney, Agent or Firm: Cesari and McKenna, LLP Rooney,
Esq.; Rita M.
Claims
What is claimed is:
1. A diffusion layer for a direct oxidation fuel cell, the fuel
cell having a protonically conductive membrane, comprising: a
substrate comprised substantially of carbon, said substrate
facilitating the transport of reactants towards a catalyst in
intimate contact with said protonically conductive membrane and
being coated with an electrically conductive layer that forms a
microporous layer on said substrate, said microporous layer having
indented channels formed therein, providing preferential flow paths
to cause fluids to travel in predetermined directions in the fuel
cell and to improve a distribution of the reactant to the catalytic
layer.
2. The diffusion layer as defined in claim 1 wherein a catalyst has
been applied to at least one side of the membrane of the fuel
cell.
3. The diffusion layer as defined in claim 2 wherein said catalyst
is applied to both sides of the membrane.
4. The diffusion layer as defined in claim 1 wherein said fluid is
substantially water, and said indented channels are formed such
that fluid is directed away from the membrane.
5. The diffusion layer as defined in claim 1 wherein said substrate
is comprised substantially of carbon cloth.
6. The diffusion layer as defined in claim 1 wherein said substrate
is comprised of at least one sheet of carbon paper.
7. The diffusion layer as defined in claim 6 wherein said substrate
is comprised of a plurality of sheets of carbon paper.
8. The diffusion layer as defined in claim 1 wherein said
electrically conductive layer is substantially hydrophobic.
9. The diffusion layer as defined in claim 8 wherein said
electrically conductive layer includes polytetrafluoroethylene and
high surface area carbon particles.
10. The diffusion layer as defined in claim 1 wherein said
substrate is placed in intimate contact with said protonically
conductive membrane.
11. The diffusion layer as defined in claim 1 wherein said indented
channels are formed in a spiral pattern.
12. The diffusion layer as defined in claim 1 wherein said indented
channels are formed in a lattice pattern.
13. The diffusion layer as defined in claim 1 wherein said indented
channels are formed in such a geometric pattern as to draw fluids
away from an active electrode area of the membrane.
14. A membrane electrode assembly comprising: a protonically
conductive, electronically non-conductive membrane; at least one
diffusion layer comprising a carbon substrate being coated thereon
with an electrically conductive layer that forms a miroporous layer
which includes indented channels formed therein, providing
preferential flow paths to cause fluids to travel in predetermined
directions in the assembly thereby providing a path through which
reactants and byproducts may be preferentially transported to
direct said reactants and byproducts in predetermined directions in
said assembly and to improve a distribution of the reactant to the
catalytic layer; and a catalyst disposed on said membrane and
catalyst forming an active area of said assembly.
15. The membrane electrode assembly as defined in claim 14 wherein
said catalyst is applied to at least one surface of the
membrane.
16. The membrane electrode assembly as defined in claim 15 wherein
said catalyst is applied to both surfaces of the membrane.
17. The membrane electrode assembly as defined in claim 14
including a plurality of diffusion layers and wherein at least one
said diffusion layer is substantially hydrophobic.
18. The membrane electrode assembly as defined in claim 14 further
comprising a membrane to which a catalyst has been applied is
disposed between two diffusion layers, at least one of which
diffusion layer provides said preferential flow path to direct
fluids away from said active area of said assembly.
19. The membrane electrode assembly as defined in claim 14 wherein
said membrane is comprised substantially of polyperfluorosulfonic
acid.
20. The membrane electrode assembly as defined in claim 14 wherein
said membrane is comprised substantially of a cation exchange
membrane based on perflouorocarbon polymers with side chain termini
of perfluorosulfonic acid groups.
21. The membrane electrode assembly as defined in claim 14 wherein
the catalyst applied to at least one surface of the membrane
contains platinum.
22. The membrane electrode assembly as defined in claim 21 wherein
the catalyst applied to at least one surface of the membrane also
contains ruthenium.
23. A direct oxidation fuel cell, comprising: (A) a membrane
electrode assembly, including: (i) a protonically conductive,
electronically non-conductive membrane electrolyte, having an anode
face and an opposing cathode face; and (ii) a catalyst coating
disposed on at least one of said anode face and said cathode face,
whereby electricity-generating reactions occur upon introduction of
fuel solution from an associated fuel source, including anodic
conversion of said fuel solution into carbon dioxide, protons and
electrons, and cathodic combination of protons, electrons and
oxygen from an associated source of oxygen, producing water; (B) an
anodic diffusion layer disposed in intimate contact with said anode
face of said membrane electrode assembly which allows said
associated fuel mixture to pass through to said anode face as fuel
is consumed at said anode, and which also allows
anodically-generated CO.sub.2 to be transported away from the anode
face of the membrane; (C) a cathodic diffusion layer disposed in
intimate contact with said cathode face of said membrane electrode
assembly and which allows oxygen to pass through to said cathode
face of said membrane electrode assembly, which cathode diffusion
layer is comprised of a carbon-containing substrate and a
microporous layer having channels formed therein to provide
preferential flow paths such that reactants and byproducts in said
fuel cell travel in predetermined directions along said channels
and to improve a distribution of the reactant to the catalytic
layer; and (D) means for collecting electric current generated in
said electricity-generating reactions to provide said electric
current to a load.
24. The direct oxidation fuel cell as defined in claim 23 wherein
said substrate of said cathode diffusion layer is comprised
substantially of carbon cloth.
25. The direct oxidation fuel cell as defined in claim 23 wherein
said substrate of said cathode diffusion layer is comprised
substantially of carbon paper.
26. A direct oxidation fuel cell system comprising: (A) a direct
oxidation fuel cell including: (i) a membrane electrode assembly,
including: a.) a protonically conductive, electronically
non-conductive membrane electrolyte, having an anode face and an
opposing cathode face; and b.) a catalyst coating disposed on at
least one of said anode face and said cathode face, whereby
electricity-generating reactions occur upon introduction of fuel
solution from an associated fuel source, including anodic
conversion of said fuel solution into carbon dioxide, protons and
electrons, and cathodic combination of protons, electrons and
oxygen from an associated source of oxygen, producing water; (ii)
an anodic diffusion layer disposed in intimate contact with said
anode face of said membrane electrode assembly which allows said
associated fuel mixture to pass through to said anode face as fuel
is consumed at said anode, and also allows anodically-generated
CO.sub.2 to be transported from the anode face of the membrane;
(iii) a cathodic diffusion layer disposed in intimate contact with
said cathode face of said membrane electrode assembly which allows
oxygen to pass through to said cathode face of said membrane
electrode assembly, and which cathodic diffusion layer is comprised
of a carbon-containing substrate and a microporous layer having
channels formed therein to provide preferential flow paths such
that reactants and byproducts in said fuel cell travel in
predetermined directions along said channels and to improve a
distribution of the reactant to the catalytic layer; and (iv) means
for collecting electric current generated in said
electricity-generating reactions to provide said electric current
to a load; (B) a fuel source; (C) fuel container and delivery
assembly coupled between said fuel source and said direct oxidation
fuel cell; (D) means for removing reactants from the fuel cell; (E)
means for removing byproducts from the membrane electrode assembly;
and (F) an electrical coupling means for connecting the fuel cell
with an external device to which is it providing power.
27. The direct oxidation fuel cell system as defined in claim 26
further comprising a coupling between an anode chamber and a
cathode chamber of said fuel cell, by which water collected from
the cathode face of the membrane is recirculated to the anode
chamber of the fuel cell.
28. The direct oxidation fuel cell system as defined in claim 27
further comprising means for mixing water recirculated from said
cathode face of said membrane with a fuel substance and means for
introducing a fuel and water mixture to the anode face of the
membrane electrode assembly.
29. The diffusion layer as defined in claim 1 wherein said
microporous layer is substantially comprised of a material that has
been allowed to form cracks extending substantially to the edges of
the diffusion layer thereby providing a pathway for fluid to be
transported away from the active electrode area.
30. The diffusion layer as defined in claim 1 wherein said
substrate and microporous layer are substantially comprised of
materials selectively chosen such that a fuel substance can pass
through said substrate, while carbon dioxide is directed through
said preferential flow paths to be released or directed to another
portion of said fuel cell.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to direct oxidation fuel cells,
and more particularly, to diffusion layers for such fuel cells.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used
to generate electricity. A variety of materials may be suited for
use as a fuel depending upon the materials chosen for the
components of the cell. Organic materials, such as methanol or
natural gas, are attractive choices for fuel due to the their high
specific energy.
Fuel cell systems may be divided into "reformer-based" systems
(i.e., those in which the fuel is processed in some fashion to
extract hydrogen from the fuel before it is introduced into the
fuel cell system) or "direct oxidation" systems in which the fuel
is fed directly into the cell without the need for separate
internal or external processing. Most currently available fuel
cells are reformer-based fuel cell systems. However, because fuel
processing is expensive and requires significant volume, reformer
based systems are presently limited to comparatively high power
applications.
Direct oxidation fuel cell systems may be better suited for a
number of applications in smaller mobile devices (e.g., mobile
phones, handheld and laptop computers), as well as in some larger
applications. Typically, in direct oxidation fuel cells, a
carbonaceous liquid fuel in an aqueous solution (typically aqueous
methanol) is applied to the anode face of a membrane electrode
assembly (MEA). The MEA contains a protonically-conductive but,
electronically non-conductive membrane (PCM). Typically, a catalyst
which enables direct oxidation of the fuel on the anode is disposed
on one surface of the PCM (or is otherwise present in the anode
chamber of the fuel cell). Protons (from hydrogen found in the fuel
and water molecules involved in the anodic reaction) are separated
from the electrons. The protons migrate through the PCM, which is
impermeable to the electrons. The electrons thus seek a different
path to reunite with the protons and oxygen molecules involved in
the cathodic reaction and travel through a load, providing
electrical power.
One example of a direct oxidation fuel cell system is a direct
methanol fuel cell system or DMFC system. In a DMFC system,
methanol in an aqueous solution is used as fuel (the "fuel
mixture"), and oxygen, preferably from ambient air, is used as the
oxidizing agent. There are two fundamental reactions that occur in
a DMFC which allow a DMFC system to provide electricity to power
consuming devices: the anodic disassociation of the methanol and
water fuel mixture into CO.sub.2, protons, and electrons; and the
cathodic combination of protons, electrons and oxygen into water.
The overall reaction may be limited by the failure of either of
these reactions to proceed to completion at an acceptable rate
(more specifically, failure to oxidize the fuel mixture will limit
the cathodic generation of water, and vice versa).
Typical DMFC systems include a fuel source, fluid and effluent
management systems, and a direct methanol fuel cell ("fuel cell").
The fuel cell typically consists of a housing, and a membrane
electrode assembly ("MEA") disposed within the housing. A typical
MEA includes a centrally disposed protonically-conductive,
electronically non-conductive membrane ("PCM"). One example of a
commercially available PCM is Nafion.RTM. a registered trademark of
E.I. Dupont Nemours and Company, a cation exchange membrane based
on perflouorocarbon polymers with side chain termini of
perflourosulfonic acid groups, in a variety of thicknesses and
equivalent weight. The PCM is typically coated on each face with an
electrocatalyst such as platinum, or platinum/ruthenium mixtures or
alloy particles. On either face of the catalyst coated PCM, the
electrode assembly typically includes a diffusion layer.
A conventional diffusion layer serves to evenly distribute liquids
and gases across the electrodes. In the case of the anode, the
diffusion layer is used to evenly distribute the fuel/water mixture
to a maximum number of contact points on the surface of the anode
so that the greatest surface area of the anode is utilized for
methanol electro-oxidation. On the cathode side, the diffusion
layer disperses oxygen so that it is more evenly introduced to the
cathode face of the PCM to promote the oxygen electro-reduction,
which produces water. In addition, flow field plates are often
placed on the surface of the diffusion layers, but are not usually
in direct contact with the coated PCM. The flow field plates
function to provide mass transport of reactants and byproducts of
the electro-chemical reactions, and the flow field plates may also
have a current collection functionality, in that the flow field
plates act to collect and conduct electrons to the load.
A typical diffusion layer may be fabricated of carbon paper or a
carbon cloth, typically with a micro-porous coating made of a
mixture of carbon powder and polytetra-fluoroethylene (Teflon, also
sometimes referred to herein as "PTFE"). The PTFE component has a
function of wet proofing in the case of a gas-supplied electrode,
but as the cell reaction proceeds, the carbon paper or carbon cloth
can become saturated with liquid water. This can be caused by
continuous water build-up in the cathode chamber of the fuel cell.
The cathode can become "flooded", in which case the cathode half of
the reaction can be compromised or even prevented. This results in
the overall performance of the cell being compromised, or
prevented.
Typically, the risk of cathode flooding is mitigated by active air
flow to remove water from the cathode layer. This, however,
increases the cost and complexity of the fuel cell system, thus
adding to the expense of manufacture, as well as introducing the
possibility of parasitic losses. In addition, it also adds volume
to a system that must meet demanding form factors.
It is also noted that when water builds up in the cathode, it not
only can cause flooding, which reduces the effectiveness of the
half reaction on the cathode side, but it also results in pressure
on the cathode face of the PCM that can weaken or compromise the
bond between the membrane (PCM) and the catalytic coating, or the
bond between the diffusion layer and the catalytic coating. Cell
performance can be reduced over the long run because these stresses
can ultimately cause separation of key fuel cell components,
preventing the effective operation thereof.
There remains a need therefore for a diffusion layer that provides
optimal gas diffusion properties, and resists flooding of the
cathode portion of the fuel cell. In addition, in direct methanol
fuel cells (DMFCs), the removal and collection of liquid water from
the cathode by means of such a modified cathode diffusion layer is
of high significance in maintaining overall water balance in the
fuel cell system. Effective collection of liquid water at the
cathode may be prerequisite to carrying just neat (pure) methanol,
rather than methanol/water mixtures, as fuel supply to the
DMFC.
It is thus an object of the invention to provide a diffusion layer
that reduces the risk of cathode flooding, and liquid water-caused
deterioration on the cathode side of the fuel cell and/or a cathode
diffusion layer that allows liquid water to be collected for use in
the direct methanol fuel cell system.
SUMMARY OF THE INVENTION
The present invention is a modified diffusion layer for use on the
cathode face of a protonically conductive membrane of a DMFC, which
is comprised of a diffusion material that has preferential flow
paths incorporated therein which redirect and remove liquid across
the diffusion layer and cause the liquid to preferentially flow, in
a predetermined manner, usually away from the PCM. The inventive
diffusion layer can thus provide a preferential path by which
liquid reactants or byproducts may be removed. By providing a means
by which fluids present on the cathode face of the fuel cell are
removed without the use of pumps or other power consuming devices,
the overall efficiency of the fuel cell is enhanced, and the
operation of the cell is improved.
In accordance with the invention, the diffusion layer includes a
substrate formed substantially of carbon, and is typically
fabricated from carbon cloth or carbon paper. The substrate is
coated with a hydrophobic microporous layer of Teflon and a high
surface area of carbon particles on at least one side. An
indentation pattern is formed to create channels, in accordance
with the invention, into the microporous layer. The patterned
channels provide paths by which fluids will preferentially travel
in the fuel cell. In the case of the cathode, the preferential
paths, or channels, direct the water to either a collection point
away from the PCM where it may be purged to the ambient
environment, or it may be recirculated to return it to the anode
side of the cell.
In accordance with the method of the present invention, the
modified diffusion layer can be fabricated by forming a substrate
substantially of carbon, such as carbon paper or carbon cloth. The
substrate is then treated by coating it with a microporous layer
comprised of Teflon-coated high surface area carbon particles. This
microporous layer is then embossed, using indentation techniques to
form therein a pattern, thus, producing preferential flow paths, to
direct the water, or other fluids, away from the membrane, and to
remove them from the active area of the cell, as desired.
In accordance with yet a further aspect of the invention, the
preferential path is established by applying a thicker wet mixture
containing Teflon and high surface area carbon particles than is
typically applied, to the carbon cloth or carbon paper. After
drying and then sintering at the glass transition temperature of
the Teflon, the face of the porous coating layer cracks, forming a
mud-cracked pattern on the resulting microporous layer. These mud
cracking patterns extend to the edges of the diffusion layer,
providing a pathway for water to be transported away from the
active electrode area.
In addition, materials may be selectively chosen for the
microporous layer and other components in the cell, to encourage
the flow of water (or other liquid byproducts and reactants) in
certain predetermined directions within the cell, to further
enhance performance of the cell.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying
drawings, of which:
FIG. 1 is a block diagram of a direct oxidation fuel cell system
with which the diffusion layer of the present invention may be
employed;
FIG. 2 is an isometric schematic drawing of a membrane electrode
assembly with which the present invention may be employed;
FIG. 3A is an isometric view of a membrane electrode assembly that
includes the diffusion layer of the present invention and
illustrates flow channels in the diffusion layer;
FIG. 3B is a top plan view of an embodiment of the diffusion layer
of the present invention in which the flow channel is of a spiral
shape;
FIG. 3C is a top plan view of an embodiment of the diffusion layer
of the present invention in which the flow channel is of a lattice
pattern;
FIG. 4A is a cross section of a membrane electrode assembly which
employs a diffusion layer constructed in accordance with another
aspect of the invention in which cracks are formed in the
microporous layer; and
FIG. 4B is a top plan view of the embodiment of FIG. 4A.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE
EMBODIMENT For a better understanding of the invention, the
components of a direct oxidation fuel cell system, a direct
oxidation fuel cell and the basic operation of a fuel cell system,
will be briefly described. One embodiment of a direct oxidation
fuel system 2 is illustrated in FIG. 1, though the invention set
forth herein may be used in a number of other system architectures.
The fuel cell system 2 includes a direct oxidation fuel cell, which
may be a direct methanol fuel cell 3 ("DMFC"), for example. For
purposes of illustration we herein describe an illustrative
embodiment of the invention with DMFC 3, with the fuel substance
being methanol or an aqueous methanol solution. It should be
understood, however, that it is within the scope of the present
invention that other fuels may be used in an appropriate fuel cell.
Thus, as used herein, the word "fuel" shall include methanol,
ethanol, propane, butane or combinations thereof and aqueous
solutions thereof, and other hydrocarbon fuels amenable to use in
direct oxidation fuel cell systems.
The system 2, including the DMFC 3, has a fuel delivery system to
deliver fuel from fuel source 4. An internal reservoir 4a may, but
need not be, utilized in conjunction with the fuel source.
Alternatively, a refillable internal reservoir may be used to store
fuel. The DMFC 3 includes a housing 5 that encloses a membrane
electrode assembly 6 (MEA). MEA 6 incorporates protonically
conductive, electronically non-conductive membrane (PCM) 7, MEA 6
also incorporates an anode diffusion layer 8 and cathode diffusion
layer 10, each of which may be coated with a catalyst, including
but not limited to platinum, a blend of platinum and ruthenium, or
other alloy with high surface area particles, which may be
supported or unsupported by carbon particles. The portion of DMFC 3
defined by the housing 5 and the anode face of the PCM is referred
to herein as the anode chamber 18. The portion of DMFC 3 defined by
the housing 5 and the cathode face of the PCM is referred to herein
as the cathode chamber 20.
As will be understood by those skilled in the art, a carbonaceous
fuel in an aqueous solution (typically an aqueous methanol
solution) passes from a fuel source 4, through the anode flow field
plate (if any) after which it enters the anode diffusion layer 8
where it is dispersed and presented to the anode aspect of the PCM
7, in a substantially uniform fashion. Similarly, an oxidizing
agent (or oxidant), preferably ambient air, is made available to
the PCM 7, via the cathode diffusion layer 10, the details of which
are described herein after in accordance with the present
invention. Those skilled in the art will recognize that flow field
plates (not shown) may be placed in contact with each aspect of the
diffusion layers 8,10 that are not in contact with the PCM 7.
Catalysts on the PCM 7 (or are otherwise present in each of the
anode and cathode chambers, 18 and 20 respectively) enable the
oxidation of the carbonaceous fuel and water mixture on the anode
face 9 of the PCM 7 forming carbon dioxide as the byproduct of the
anodic reaction, and releasing protons and electrons from the
hydrogen atoms in the fuel and water mixture. Upon the closing of
an external circuit, the protons pass through the PCM 7, which is
impermeable to the electrons. The electrons seek a different path
to reunite with the protons and travel through a load and, thus,
provide the electrical power from the fuel cell 3. The
electrochemical reactions are as follows:
As stated before, the second half of the reaction occurs in the
cathode and it is described in the above Equation 2. More
specifically, water is produced at the cathode face of the PCM.
Under some operating conditions, so much water is created or passes
through the PCM, that the catalyst diffusion layer 10 and/or the
cathode catalyst layer can become flooded causing the DMFC to cease
functioning.
Referring now to FIG. 2, the diffusion layer of the present
invention, which provides a solution to this problem, will be
described in detail. A protonically-conductive membrane PCM 7 has a
cathode face 11, which is coated with a catalyst layer 30. A
diffusion layer 10 is placed contiguous to the catalyst layer 30.
The diffusion layer 10 is fabricated as a substrate 44 formed
substantially of carbon, such as carbon cloth or carbon paper. A
hydrophobic microporous layer comprised of Teflon-coated high
surface area carbon particles is typically applied to the substrate
44, forming a microporous backing layer 48. The microporous backing
layer 48 is in intimate contact with the catalyst coated membrane
7, in order to minimize resistance to the flow of electrons across
the fuel cell.
In accordance with one aspect of the present invention, the
diffusion layer is embossed with a pattern, as illustrated with
greater detail with reference to FIGS. 3A-3C. More specifically,
the cathode diffusion layer 10 has embossed therein flow channels,
which create a preferential flow path upon which water (or a liquid
containing water, but which may also include a fuel solution) will
travel away from the cathode face 11 of the membrane 7. Potential
paths include but are not limited to those shown in the isometric
view of FIG. 3A, the flow channels 50 and 52, for example, allow
the water to flow away from the center of the membrane and
consequently the fluids may be collected and recirculated, or
eliminated from the system. As shown in FIG. 3B, the flow channel
can be in a spiral pattern 22. As shown in FIG. 3C, a top plan
view, the flow channels can be of a lattice layout having
substantially linear portions, such as flow channel portions 22 and
24. These embossments will create a preferential liquid flow path
to direct fluids as desired in the cell, and in one exemplary
embodiment, will direct liquid water away the cathode diffusion
layer to prevent and resist flooding of the cathode and separation
of the cathode components of the fuel cell. It should be understood
that many other geometric patterns may be formed as flow channels
in the diffusion layer, while remaining within the scope of the
present invention.
While not limiting to the invention, the flow channels may also
minimize buildup of pressure by allowing liquid that accumulates at
the interface between the cathode catalyst and the cathode
microporous backing layer 48, (also referred to herein as a
microporous layer), an outlet, so that hydrostatic pressure that
could be created by such liquid does not build up. Instead, in the
presence of the embossed flow channels the liquid is removed by the
small hydrostatic pressure build up.
In addition, the flow channels may also provide an outlet to
release the water built to up from the cathodic reaction and the
accompanying water transport through the membrane. Water generated
in the active electrode area, where the hydrophobic microporous
layer is in direct and intimate contact with the catalyst layer,
will be pushed away from the active electrode area and into the
flow channels by the hydyrostatic pressure generated by the
capillary force of the hydrophobic microporus layer. It thus opens
the one-way transportation of oxygen from the air to the cathode
catalyst layer without the egress flow of water encountered with a
non-improved backing. Without water accumulation between the
cathode catalyst layer and microporous layer or between the
catalyst layer and membrane by using the improved cathode backing,
the risk of cell component delamination may be eliminated. The
hydrostatic pressure that drives water into the channels formed in
the microporous layer may also be used to collect and direct the
water from the cathodic reaction from the cathode compartment to
the anode compartment passively, thus minimizing excessive water
loss from the cathode. As a result, a higher power pack energy
density can be achieved by carrying more methanol and less water as
the fuel mixture.
The flow channels direct the water either to a collection point
such as collection point 60 (FIG. 3C) or it may be eliminated into
the ambient environment, or it may be returned to the anode side of
the cell or of another cell as shown in FIG. 1. Those skilled in
the art will recognize that the invention may also be used where a
stack or other assembly containing more than one fuel cell is
connected.
The pattern on the diffusion layer can be formed not only by
embossing, but by other mechanical means that are appropriate in
the particular application in which the invention is being employed
such as milling or casting the microporous layer to form the flow
channel pattern into the component in accordance with the present
invention. The milled or cast layer would then be bonded or
otherwise attached to the substrate of the diffusion layer using
methods known to those skilled in the art.
Another aspect of the invention will be described with reference to
FIGS. 4A and 4B. More specifically, in accordance with yet a
further aspect of the invention, the preferential path is
established by applying a thicker wet mixture containing Teflon and
high surface area carbon particles, to the carbon cloth or carbon
paper than is typically applied. After drying, then sintering at
the glass transition temperature of the Teflon, the face of the
porous coating layer cracks, forming a mud-cracked pattern 400 on
the resulting microporous layer as shown in FIG. 4B. These mud
cracking patterns can extend to the edges of the diffusion layer
402, 404, providing a pathway for water to be transported away from
the active electrode area, to a point that is away from the active
area of the PCM. The cracks would not extend into the membrane, but
instead would extend from the surface of the microporous backing
layer, partially towards the surface of the substrate.
A commercially available diffusion layer, such as ELAT, may also be
modified by applying an additional layer of Teflon coating (or an
additional layer of the material used to fabricate the microporous
layer) to one side of said diffusion layer, then causing such layer
to crack, using the same methods as set forth above. Whether the
diffusion layer is fabricated from raw materials or is fabricated
by modifying an existing diffusion layer, the cracks will, at most,
penetrate the diffusion layer only to the depth of the substrate,
as noted. The microporous layer opposite the face of the diffusion
layer into which the preferential flow channels are established
remains entirely intact, thus providing a diffusion layer with a
preferential path without compromising the structural or conductive
integrity of said diffusion layer. Alternate methods of cracking
the microporous layer are within the scope of the invention. It
should be understood that this embodiment may also be easier to
manufacture, while still providing the benefits of removal of water
from the active cathode area and protecting the PCM.
Performance may also be enhanced by adding hydrophobicity or
hydrophilicity character to the appropriate materials in the cell
which would also encourage and facilitate water removal from the
cell or cathode and to encourage other byproducts to travel in such
a direction so as to allow the energy generating reactions of the
fuel cell system to proceed more efficiently to completion.
The method of the present invention includes the steps of making an
improved diffusion layer by fabricating a substrate substantially
of carbon, such as fabricating the substrate out of carbon paper or
carbon cloth. The substrate is then treated by coating it with an
electrically conductive layer, which is substantially hydrophobic,
but permeable to gases, thus forming a microporous layer over the
substrate. The microporous layer may be a composite of Teflon and
high surface area carbon particles. This microporous layer is then
embossed, by high pressure indentation techniques to form therein a
pattern, thus, producing a preferential flow path, to direct water,
or other liquids fluids, away from the membrane, and remove them
from the active area of the cell, as desired. Instead of embossing,
other mechanical techniques may be used such as milling and casting
to form a layer having flow channels therein.
The method of the present invention for fabricating a liquid
evolving, which may also be referred to herein as a liquid removing
diffusion layer, may be further understood with reference to the
accompanying example, which is illustrative only and not limiting
to the invention.
EXAMPLE
A diffusion layer was fabricated by employing a substrate formed
from a sheet of ELAT diffusion backing, commercially available from
the E-Tek division of De Nora N.A., 39 Veronica Avenue, Somerset,
N.J. 08873, measuring about 3.162 cm by 3.162 cm. This diffusion
layer was comprised of a carbon cloth substrate with a microporous
layer comprised of Teflon-bonded high surface area carbon
particles. A lattice like pattern was embossed into the diffusion
layer using pressure of 10,000 pounds per square inch, the pattern
extending to the edges of the diffusion layer. The embossed
diffusion layer was then placed in intimate contact with a catalyst
coated membrane for use in the membrane electrode assembly of a
direct methanol fuel cell.
The microporous layer tends to form a hydrophobic barrier adjacent
the catalyst coated PCM, however, water can still build up near the
cathode face of the fuel cell. The embossment in the cathode
diffusion layer causes such water to travel along the preferential
flow paths and be removed from the cathode, thus reducing the risk
of cathode flooding which could limit the flow of oxygen to the
cathode face, and limiting cell performance. It should be
understood by those skilled in the art that the diffusion layer of
the present invention allows gaseous reactants to reach the
membrane while removing liquid byproducts from the membrane. The
diffusion layer, on the cathode side, allows oxygen to diffuse
through the microporous layer to the cathode catalyst layer, while
the water produced in the cathode half reaction as well as water
crossing through the membrane is directed away from the membrane.
In addition, the diffusion layer of the present invention protects
the cathode side of the membrane by reducing hydrostatic pressure
that can otherwise cause delaminating problems.
It should be understood that the diffusion layer of the present
invention assists in prevention of cathode flooding without the
application of external force or energy. More specifically, the
present invention provides a method of water removal that does not
require active drying of the cathode layer.
Thus, it should be understood that the diffusion layer of the
present invention provides many advantages for use with a low
temperature direct oxidation fuel cell, where water tends to build
up at the cathode catalyst and is required to be collected and
redirected to maintain water balance. While we have described the
diffusion layer with respect to the cathode, there may also be
instances in which a patterned diffusion layer that includes
preferential flow paths may be advantageously employed in the anode
chamber of the fuel cell, while remaining within the scope of the
present invention.
The foregoing description has been directed to specific embodiments
of the invention. It will be apparent however that other variations
and modifications may be made to the described embodiments with the
attainment of some or all of the advantages of such. Therefore, it
is the object of the appended claims to cover all such variations
and modifications as come within the true spirit and scope of the
invention.
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